Chlamydia trachomatis (CT) infection in the lower genital tract can ascend to the upper genital tract, causing pathologies such as hydrosalpinx, a hallmark for tubal infertility in women. However, the mechanisms on how CT ascends and induces hydrosalpinx-causing inflammation in women remain unknown. Chlamydia muridarum (CM) have been extensively used for studying the mechanisms of CT pathogenesis and immunity because intravaginal inoculation with CM in some mice can induce hydrosalpinx similar to that observed in women. We have optimized the mouse model and screened 11 different strains of mice for hydrosalpinx induction by CM, which has led us to conclude that chlamydial ascension and induction of tubal inflammation are two major determinants of chlamydial pathogenicity. The chlamydial plasmid is a key virulence factor since plasmid-deficient CM can no longer induce hydrosalpinx in mice and CT serovars lacking the plasmid are attenuated in monkeys and mice. The chlamydial plasmid encodes 8 open reading frames (pORF1-8 or Pgp1-8). Pgp3 or plasmid glycoprotein 3 is immunodominant and both associated with chlamydial outer membrane complex and secreted into the host cell cytosol. Pgp3 forms a stable trimer and is a major virulence factor encoded by the plasmid since Pgp3-deficient Chlamydia phenocopied their plasmid-free counterparts. We hypothesize that Pgp3 plays critical roles in both ascension and tubal inflammation. This hypothesis is consistent with our recent preliminary data that Pgp3 can neutralize the antichlamydial activity of host extracellular effectors. Extracellular mucosal effectors may be able to prevent chlamydial ascension by inactivating chlamydial elementary bodies (EBs) released extracellularly during cell-to-cell spreading, which may select Chlamydia for acquiring virulence factors such as Pgp3 to neutralize these same effectors. The Pgp3 proteins secreted into and stored in the cytosol of the infected cells can be released extracellularly upon cell lysis to neutralize the mucosal effectors so that the simultaneously released progeny EBs can find new target cells for achieving ascension. To test the above hypothesis, we will first establish the roles of Pgp3 in ascending infection and tubal inflammation. Although Pgp3 seems to be essential for both chlamydial ascension and induction of tubal inflammation, the precise role of Pgp3 in each process has not been defined because ascension and inflammation can interfere with each other. In the current grant, we will quantitatively determine the precise contributions of Pgp3 to CM infectivity, ascension and tubal inflammation so that the mechanisms of Pgp3 can be investigated. We will further elucidate the mechanisms by which Pgp3 promotes chlamydial ascension and tubal inflammation. Accomplishing the proposed experiments will not only allow us to understand chlamydial pathogenic mechanisms but also provide knowledge for improving diagnosis, treatment and prevention of chlamydial infection and pathogenicity and for developing reagents to modify inflammation under various conditions.